The present invention relates to a semiconductor memory device and a manufacturing method thereof, and more particularly to a magneto-resistive random access memory (MRAM) and a manufacturing method thereof.
Semiconductor memory devices include Dynamic Random Access Memory (DRAM) devices. However, the DRAM devices have limitations such as being scaled down and maintaining a capacitance of capacitors that store data in the scale-down devices. To overcome the limitations of conventional DRAM devices, semiconductor memory devices structures have been developed. As one type of memory device, a Magneto-resistive Random Access Memory (MRAM) device uses the characteristics of Tunneling Magneto-Resistance (TMR). The TMR is a magneto-resistive effect that occurs in a magnetic tunnel junction (MTJ).
The MRAM device is a non-volatile memory device where data is stored by magnetic storage elements having different resistances according to a magnetic field changed by magnetic polarities of two ferromagnetic plates forming the MTJ. The MTJ is a component including two ferromagnetic plates separated by an insulating layer. A first ferromagnetic plate is a pinned layer (PL) set to have a magnetic polarity, and a second ferromagnetic player is a free layer (FL) having a polarity changed by a current passing through the layers.
When electrons passing through a first plate of the two ferromagnetic plates penetrate into the insulating layer serving as a tunneling barrier, the probability that the electrons penetrating into the insulating layer changes based on the polarity of a second plate of the two ferromagnetic plates. If the polarities of the two ferromagnetic plates are parallel (the same direction), the tunneling current is maximized. Otherwise, if the polarities of the two ferromagnetic plates are opposite, the tunneling current is minimized. The state of the tunneling current indicates what information is stored in the MTJ.
The MRAM device typically uses a Spin Transfer Torque (STT) technique to write data therein. The STT technique uses spin-aligned (“polarized”) electrons to directly torque domains. The torque will be transferred to a nearby ferromagnetic plate, according to an effect that may modify the orientation of a ferromagnetic plate in a tunnel magnetoresistance or spin valve using a spin-polarized current. When the spin-polarized current flows into the ferromagnetic plate, if the magnetic orientation of the ferromagnetic plate is not the same as the polarity of the current, the magnetic orientation is aligned to the polarity of the current so that the data can be written.
In the MTJ included in the MRAM device, when electrons flow from the pinned layer to the free layer, the magnetic orientation of the free layer is aligned with that of the pinned layer by the electrons having spin aligned with the polarity of the pinned layer. Thus, the MTJ can store a first type of data. Otherwise, if electrons flow from the free layer into the pinned layer, spin accumulation occurs at boundary areas of the pinned layer and the free layer. Thus, the magnetic orientation of the free layer is oppositely aligned with that of the pinned layer so that a second type of data can be stored in the MTJ.